It has been proposed to cultivate biological material such as algae on a large scale to provide an efficient and sustainable source of nutrients, chemical feedstocks and/or fuel, e.g., biogas, biocrude or biodiesel. The advantages of algae are that it can be cultivated in salt water, brackish water or fresh water and represents a source of biomass and oils that is potentially carbon neutral, that takes up little or no arable land, and will not displace conventional food crops from the marketplace. Despite many previous attempts, unfortunately, it has proved difficult to cultivate algae in an economical and cost effective manner.
One previous approach has been to try to harvest both biomass and oils from an algae crop. For example, U.S. Pat. No. 8,287,732 discloses a method where algae is cultivated, oils extracted for conversion to biofuel, and then the residual biomass hydrolyzed and anaerobically digested to extract nutrients such as carbon dioxide and compounds containing nitrogen and phosphorous to be recycled back into the algae cultivation process. Recovery and reuse of nutrients from biomass remaining after lipid extraction by hydrolysis or anaerobic digestion is also described by this patent.
U.S. Pat. No. 7,977,076 describes a multi-stage algae cultivation process for producing biodiesel and/or bioethanol. In this cultivation process, a first type of algae was cultivated autotrophically (under light) and then converted into feed for a second type of algae that produces oil, wherein the oil is extracted and converted to biodiesel. Both the '732 patent and the '076 patent are incorporated by reference herein.
Nevertheless, obstacles remain to achieving cost-effective integrated algae cultivation to produce biothels and the like. In particular, the control of temperature, carbon dioxide (CO2) and oxygen (O2) levels during cultivation remains a challenge. Algae requires relatively stable temperatures and gas levels to maintain optimal growth conditions. While CO2 is a desirable nutrient to support photosynthetic growth, CO2 must be held within an optimal percentage of the circulating gas mixture, e.g., from about 10-13 (vol %) have found to be convenient for Chlorella sp. growth (Hulatt, C. J., et al., 2012. Bioenergt Resources 5: 669-684; Douskova, I., et al., 2008 “Microalgae as a means for converting flue gas CO2 into biomass with a high content of starch.” Bioenergy: Challenges and Opportunities International Conference and Exhibition on Bioenergy. Apr. 6-9, 2008. Universidade do Minho, Guimarães, Portugal). In addition, O2 is a byproduct of photosynthesis, but O2 levels in the aqueous medium an the algae cultivation system should be kept below about 400% of the air saturation value (Chisti, Y. 2007 Biotechnology Advances 25: 294-306.) in order to avoid deleterious effects for algae cultivation in a closed system.
Thus, there remains a longstanding need in the art for solutions that provide for the economically efficient cultivation of algae, while addressing these previous shortcomings.